A continuing goal in semiconductor device fabrication is to create increasing densities of circuitry on semiconductor real estate. Such goal is realized through ever-decreasing dimensions of semiconductor circuit elements. For instance, in the early 1970's a typical gate length of a field effect transistor gate in a dynamic random access memory (DRAM) device was on the order of from 5 to 6 micrometers, and polysilicon was utilized as a sole conductive material of the gate. Advances in DRAM generation of the late 1980's reduced the gate length to approximately one micrometer. However, it was found that word line resistance was too high if conductively doped polysilicon was utilized as the sole conductive component of a gate line, and accordingly silicide (such as tungsten silicide, molybdenum silicide or titanium silicide) was deposited over the polysilicon. The term “polycide” was coined to describe a stack of gate materials which comprised conductively doped polysilicon having a silicide thereover.
Technological advances of the 1990's reduced the gate length to less than 0.2 micrometers. It was found that the resistance of polycide materials was too high for such gates, and accordingly procedures were developed to provide a metal to replace the silicide of the polycide structure. Exemplary metals utilized are tungsten, molybdenum and titanium. Such gates would be considered modern structures in current technology.
FIG. 1 shows a semiconductor wafer fragment 10 comprising a field effect transistor 12 having such a gate structure. More specifically, wafer fragment 10 comprises a substrate 14 having a gate structure 16 formed thereover. Gate structure 16 comprises a gate oxide layer 20 (which typically comprises silicon dioxide), a conductively-doped-semiconductive-material layer 22 (which can comprise silicon and germanium, and which typically comprises conductively doped polysilicon), a conductive diffusion barrier layer 24 (which typically comprises a metal nitride, such as, for example, WNx, TiN), a metal layer 26 (which can comprise, for example, tungsten, molybdenum or titanium), and an insulative cap 28 (which can comprise, for example, silicon nitride or silicon dioxide).
Semiconductive substrate 14 can comprise, for example, conductively doped monocrystalline silicon. To aid in interpretation of the claims that follow, the terms “semiconductive substrate” and “semiconductor substrate” are defined to mean any construction comprising semiconductive material, including, but not limited to, bulk semiconductive materials such as a semiconductive wafer (either alone or in assemblies comprising other materials thereon), and semiconductive material layers (either alone or in assemblies comprising other materials). The term “substrate” refers to any supporting structure, including, but not limited to, the semiconductive substrates described above.
Gate structure 16 has opposing sidewalls 30, and insulative spacers 32 are formed along such opposing sidewalls. Insulative spacers 32 can comprise, for example, silicon nitride.
Source/drain regions 18 formed proximate gate structure 16, and a channel region 19 is defined beneath gate structure 16. Spacers 32 can be utilized during formation of source/drain regions 18 to space an implant of a conductivity-enhancing dopant from sidewall edges 30, and to thereby control a location of heavily doped source/drain regions 18 relative to sidewalls 30. Lightly doped diffusion regions are formed beneath sidewalls 32, and between heavily doped source/drain regions 18 and channel region 19, to define graded junction regions 33. The lightly doped diffusion regions are frequently formed prior to provision of sidewalls 32.
A problem can occur in utilizing the field effect transistor structure 12 of FIG. 1 in DRAM devices. DRAM devices normally operate with a wordline voltage in excess of power supply voltage (a so-called boosted wordline). Accordingly, transistor gates utilized in gated DRAM structures are exposed to larger electric fields than in other devices, and are more subject to breakdown and failure. Also, DRAM retention time depends on the storage node junction leakage, which in turn can be affected by the electric field at intersecting corners of the gate and the drain junction. The electric field between the gate and the drain junction often induces more junction leakage and is frequently referred to as Gate Induced Drain Leakage (GIDL). It is therefore desirable to have a thickened gate oxide region at the corner of the gate and the drain to reduce the electric field, and hence the leakage.
One of the techniques utilized to enhance integrity of transistor gates is to oxidize a portion of a semiconductive material substrate proximate the gate to form small “bird's beak” structures beneath sidewall edges 30. Such technique is illustrated in FIG. 2 wherein wafer fragment 10 is illustrated at a processing step subsequent to the formation of gate structure 16, but prior to formation of spacers 32 and source/drain regions 18. An upper surface of semiconductive material wafer 14 has been oxidized to form a silicon dioxide layer 34 which connects with gate oxide 20. Silicon dioxide layer 34 comprises small bird's beak regions 36 which extend beneath sidewalls 30. Silicon dioxide layer 34 also extends along a portion of sidewall 30 corresponding to the sidewall edges of semiconductive-material layer 22, as such edges are oxidized during the oxidation of the upper surface of semiconductive material 14.
A problem which occurs with the processing of FIG. 2 is that sidewall edges of metal layer 26 can be oxidized during the oxidation of semiconductive material 14. Oxidation of metal layer 26 forms metal oxide regions 38. The volume expansion associated with the formation of metal oxide regions 38 can cause lifting of the metal lines, which can result in failure of field effect transistor structures incorporating gate structure 16.
Among the techniques which have been utilized to -avoid oxidation of the metal edge are wet hydrogen oxidation, and the utilization of silicon nitride or silicon dioxide to protect the edges. Additionally, silicon oxynitride has been utilized to cover edges of the metal material in the gate stack prior to oxidation of an upper surface of semiconductive material 14.
The above-described problems are not limited to field effect transistor technologies. The problems can also occur in stacks utilized for other memory devices, such as, for example, the gate stacks utilized in flash memory devices. FIG. 3 illustrates a semiconductor wafer fragment 50 comprising a semiconductive material substrate 52, and a flash memory device gate stack 54 formed over substrate 52. Substrate 52 can comprise, for example, monocrystalline silicon lightly doped with a p-type background dopant. Gate stack 54 comprises a gate oxide layer 56 (which can comprise silicon dioxide), a floating gate 58 (which comprises semiconductive material, which can comprise Si and Ge, and which typically comprises conductively doped polysilicon), an intergate dielectric layer 60 (which can comprise silicon dioxide), a conductively-doped-semiconductive-material layer 62 (which can comprise conductively doped polysilicon), a barrier layer 64 (which can comprise a metal nitride), a metal layer 66 (which can comprise tungsten, titanium or molybdenum), and an insulative cap 68 (which can comprise silicon nitride). FIG. 3 also shows an oxide layer 69 over substrate 52, and Lightly Doped Diffusion (LDD) regions 71 implanted beneath oxide layer 69 and proximate gate stack 54. LDD regions 71 can be formed by, for example, implanting n-type conductivity enhancing dopant (such as phosphorus or arsenic) into substrate 52.
Note that layers 60, 62, 64, 66 and 68 comprise a stack identical to the stack utilized in gate structure 16. Accordingly, oxidation of semiconductive material substrate 52 can lead to problems similar to those discussed above regarding oxidation of semiconductive material 14. Specifically, oxidation of semiconductive material 52 can be accompanied by oxidation of sidewall edges of metal layer 66 which can cause failure of a circuit device incorporating stack 54.
The above-described FIGS. 1-3 illustrate cross-sectional views through the described stacks of conductive and insulative materials. Such cross-sectional views are utilized to illustrate various layers within the stacks. An alternative description of the stacks of FIGS. 1-3 is to refer to the stacks as portions of patterned wordlines. In such alternative description, it is to be understood that the stacks can be portions of lines extending across the respective semiconductor material substrates (i.e., the stacks can be patterned in the shape of lines). Source/drain regions will be provided at various intervals along the lines, and the lines will thus have transistor gate regions functioning as gating structures between respective pairs of source/drain regions.
It would be desirable to develop alternative methods of forming gate stacks and wordlines.